Power systems often include a power converter that is configured to convert direct current (DC) power into a suitable power for application to a load, such as a generator, motor, electrical grid, or other suitable load. For instance, a power generation system can include a power converter for producing alternating current (AC) power at a grid frequency (e.g. 60/50 Hz) suitable for application to an electrical grid. In solar or batter energy systems, the solar or battery source can provide direct current power to the power converter, which can then be converted to suitable AC output power for the electrical grid. In applications requiring AC to AC conversion, such as wind energy applications, the power converter can include a two stage power converter to provide AC to DC to AC conversion.
To provide increased output power capability, a power converter can include a plurality of bridge circuits coupled in parallel with one another. Each bridge circuit can include a plurality of switching elements (e.g. insulated gate bipolar transistors (IGBTs)). The pulse-width-modulation (PWM) of the switching elements can be controlled according to a desired switching pattern to provide a desired output of the power converter. The use of switching elements, such as IGBTs, in a power converter can produce undesirable high frequency components in the output power provided by the power converter. To reduce these undesirable high frequency components, one or more inductive elements can be used in conjunction with the bridge circuits to filter the high frequency components.
Timing differences can exist between the switching of the switching elements in the parallel bridge circuits. These timing differences can result from, for instance, different delay times provided by optoisolators and other components of driver circuits used to drive the switching elements. In addition, timing differences can result from controlling the parallel bridge circuit according to a switching pattern that provides for switching of the switching elements in a manner out of phase with one another, such as according to an interleaved switching pattern. The timing differences can induce a voltage across an inductive element effectively coupled between the plurality of parallel bridge circuits, resulting in a circulating current between the parallel bridge circuits.
The circulating current between parallel bridge circuits can cause a current imbalance between the parallel bridge circuits. The imbalance in current can result in a difference of temperatures in the switching elements used in the parallel bridge circuits, such as a difference in junction temperature of IGBTs used in the switching elements. This reduces the overall output power capability of the power converter as the total output current capability is limited by the switching element with the highest temperature.
Thus, a need exists for a control scheme for reducing current imbalance among parallel bridge circuits in a power converter used in power systems.